CONTAMINATION ISSUES

Management of copper impurities in the wave soldering process.

Impurities such as copper are introduced into the wave soldering process as a result of printed circuit boards and their components coming into contact with the solder contained in the solder bath. Since the tracks on the pcb are primarily copper (as are a majority of the component legs) then it follows that the most significant contaminant is likely to be copper. The impurities contained in the solder will reach an equilibrium determined by the following:

        Wa = Wrj + Wrd

where

Wa is the weight of impurity added to the solder in unit time
Wrj is the weight of impurity removed in the joints of product passing through the bath
Wrd is the weight of impurity removed in the dross

From this it can be seen that if less dross is removed by weight then there will be a corresponding increase (but not necessarily linear) in the accumulation of weight of impurity retained in the solder bath. From this we can expect any reduction in the amount of solder lost in the dross removal process to result in an increase in the level of impurity contained in the solder bath. Consider therefore the following comparison:

Standard atmospheric soldering

EVS/Atmospheric soldering

Closed Nitrogen soldering

Based therefore on the above arguments, it can be seen that impurity (copper) build up in the solder bath would be quickest and reach the highest level of equilibrium with closed loop Nitrogen soldering, with lower levels being reached and sustained by both EVS and non EVS processes.

Indications are that increases in copper levels of 100% and 60% are being seen for nitrogen and EVS soldering respectively.

Should the level of copper impurity build up become unacceptable in either atmospheric soldering option, then there are easy methods (not chemical based) available to reduce the concentration of copper in the solder bath which we would be pleased to discuss with you.